Is it normal for my mother to defend him all the time?

When i was a little girl my mother used to leave me my older brother & 2 little brothers alone in the house while she went to work. The older brother sexually abused me & raped my little brother who was only 5 at the time. We told my mother & she wasn't botherd. Anyways since then the older brother went on to brutally murder a 75yo man. He's since been released and all my mother does it defend him. She defends him to the point of lying to the police for him. Since he was let out of jail in 2012 he has beaten me up bust my head open strangled me & help me captive. Mother lied for him so he never got jailed. He's attacked an ex gf whilst she was pregnant causing her to lose her baby. Yet again nothing happend due to mother giving him an alibi. Even tho it was 25yrs back since the abuse he still has paedophile tendancies sending pictures of his penis to my 5yo neice. (she was playing a game on her moms phone & he was aware of this so sent the picture) no matter what this vile human being does mother will always supply him with an alibi. Why does she do this? & Is It Normal for a mother to do this?. NO MATTER WHAT HE DOES MY MOTHER CAN SEE NO WRONG